This invention relates to the bending of sheets of thermoplastic material, especially glass. Many techniques for bending glass sheets are known. Perhaps the simplest technique is to heat a glass sheet while supported on a contoured mold and to permit the glass sheet to sag into conformity with the mold. However, it is often desirable to speed the process by applying a mechanical bending force such as by pressing the glass between a pair of contoured molds.
A particularly advantageous press bending process is shown in U.S. Pat. No. 3,846,104 to S. L. Seymour wherein a horizontally oriented glass sheet is heated and lifted by a lower bending mold into contact with an upper bending mold where it is retained by vacuum while the lower bending mold retracts. Then a tempering ring receives the bent glass sheet and conveys it from the bending station into a tempering station. That arrangement is especially advantageous in that it provides contoured support for the bent glass sheet during the tempering step and frees the bending station for initiating the next bending cycle while tempering of the first glass sheet is being carried out. A drawback to such an arrangement is that three major components of the apparatus, the upper and lower forming molds and the tempering ring, must all be custom fabricated for each different shape produced on such a bending and tempering line. It would be desirable not only to reduce the cost of fabricating these elements with each shape change, but also to reduce the down time of the bending and tempering line necessitated by the installation of these elements with each product change.
In U.S. Pat. No. 3,713,799 to H. A. McMaster, a similar arrangement is disclosed, but in which the lower shaping mold serves to carry the bent glass sheet into the tempering station, thus delaying the commencement of the next bending cycle until the lower bending ring deposits the glass sheet in the tempering station and returns to the bending station. Likewise, in this arrangement a product change requires a major retooling of the bending station since the upper and lower forming molds as well as the gas support block into which the lower forming mold recesses, all must conform to the shape of the glass sheets being processed. A similar arrangement in U.S. Pat. No. 3,573,889 to H. A. McMaster et al. has the same drawback.
U.S. Pat. Nos. 3,507,639 to S. L. Seymour and 3,676,098 to H. R. Hall both show horizontal press bending arrangements wherein only two elements, the upper and lower bending molds, need to be custom fabricated for each glass shape being produced. It would be desirable to reduce the number of custom made parts even further. Furthermore, in both of these arrangements the edges of the bent glass sheets are not supported as they are conveyed from the bending station into the tempering station.
U.S. Pat. No. 3,476,540 to Ritter et al. discloses a glass bending arrangement whereby the inertia of a single vertically rising lower bending mold effects the bending. Disadvantageously, the bent glass sheets must pass without edge support along a roller conveyor into the tempering zone.
U.S. Pat. No. 3,600,150 to Rougeux shows a glass bending arrangement wherein a heat-softened glass sheet is slipped from a roller conveyor onto a flexible hammock and thereafter press bent between upper and lower forming molds. The purpose of the flexible hammock is to support the glass sheet initially out of contact with the rigid shaping mold surfaces. It is apparent that a major reconstruction of the apparatus would be required when a change in the glass shape is desired.
While the drop forming method and apparatus disclosed in the parent application Ser. No. 960,404 overcome many of the problems mentioned above, additional versatility in shaping glass sheets with such a process to a wider variety of shapes, especially relatively deep bends, would be highly desirable. One difficulty encountered in the drop forming process is that when shapes having a relatively deep bend in one direction are made, the glass is sometimes over-bent in the transverse direction. The present invention is an improvement in the drop forming process which helps overcome this problem.